Permeability is a critical factor in the maintenance of abnormal fluid pressures. The permeability of "tight" geologic materials is difficult to measure; it is particularly difficult to measure in situ. Both the laboratory and in-situ methods need special techniques when the permeabilities are less than 10^-10 cm/sec (10^-7 Darcys). Permeability can be measured (1) on rock samples in the laboratory; (2) in situ, utilizing well test procedures; and (3) on a regional scale, utilizing a hydrodynamic analysis of the entire system.

Generalizations from the results of a comparison of these techniques for Cretaceous shale in South Dakota can be used to summarize our knowledge of clay and shale permeability.

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